Blooming is a well known phenomenon that occurs in solid-state image sensors when the number of photocarriers generated by the incident radiation exceeds that of the storage capacity of the element, or pixel. These excess carriers then spill over, or "bloom", into adjacent photosites thereby degrading the integrity of the image. Many types of structures have been proposed in the past, such as U.S. Pat. No. 5,130,774 for example, which provide sinks for these excess carriers either laterally or vertically adjacent the photodetector elements. The advantages and disadvantages of both types have also been discussed.
It is important to maintain high quantum efficiency and charge capacity. Therefore, antiblooming structures should not take up so much space that there is a degradation in the quantum efficiency and charge capacity of the device. Many conventional antiblooming structures are inherently subject to level-to-level misalignment. The extra space taken up within these antiblooming structures to allow for the level-to-level misalignment can result in a reduction in performance of the sensor.
Some of the more recent disclosures are contained in U.S. Pat. No. 5,349,215 issued to Anagnostopoulos et al. (hereinafter referred to as Anaanostopoulos); U.S. Pat. No. 5,130,774 issued to Stevens et al. (hereinafter referred to as Stevens); U.S. Pat. No. 5,118,631 issued to Dyck et al. (hereinafter referred to as Dyck); and U.S. Pat. No. 4,593,303 also issued to Dyck et al. (hereinafter referred to as Dyck "303"); describe relatively modern approaches at antiblooming structure design. Another important factor in the performance of these antiblooming structures is the length (in microns) of the blooming channel's barrier region. The length of the blooming channel's barrier regions in Anaanostopoulos and Stevens, are unaffected by alignment, but they are not self aligned to the drain. The extra amount of area that must be added to compensate for misalignment becomes an important factor for small pixel size devices. Dyck discloses a self aligned structure, but offers little flexibility in adjusting the length of these barrier regions since this length depends on lateral diffusion of the barrier-region implant.
The length of the barrier regions in this structure is only about 0.5 .mu.m. Although this is very short, and therefore conserves the pixel's surface area, it makes the structure susceptible to the so-called DIBL (drain-induced, barrier lowering) effect. This effect can reduce the antiblooming barrier height dramatically thereby resulting in reduced charge capacity and hence, lower dynamic range. This also makes the barrier height sensitive to the LOD voltage. Hence, this voltage may need to be adjusted on a part-to-part basis due to process variations. Also, changing the length of this region requires changing the process (by varying Dt).
As can be seen by the foregoing discussion, there remains a need within the art for an antiblooming structure design that can offer the advantages of self alignment as well as solving the problems associated with short antiblooming barrier lengths.